Manufacture of ammonium sulphate



Patented Got. 15, 1940 UNITED STATES PATENT OFFICE Arthur D. Little, Incorporated,

Cambridge,

Mass, a corporation of Massachusetts No Drawing. 7 Application September 7, 1938,

I Serial No. 228,797

9 Claims.

This invention relates to the production of useful products from ammonia and ferrous sulphate, and more particularly to the treatment of spent pickling liquor with by-product ammonia to recover ammonium sulphate and iron compounds.

In conventional procedures for pickling iron and steel bars, sheets, and other shapes, it is customary to subject these objects to a bath of hot, dilute sulphuric acid. The bath is reused a number of times, with addition of further amounts of acid as required, until it becomes too dirty or too saturated with salts to be satisfactory for further use. The resultingspent bath of pickling liquor, consisting essentially of an aqueous solution of ferrous sulphate and free sulphuric acid, must then be disposed of.

In the past, it has been common practice either to dump the spent pickling liquor directly into rivers or streams, to neutralize with lime and discharge to streams, or to neutralize with lime, settle the resulting slurry, decant the water to a stream and haul the slurry to a dry dump. Because of the considerable amount of salts and acids in the spent liquor the practice of dumping into streams is not only quite undesirable from the standpoint of stream pollution, but in many cases forbidden. Furthermore, it is not only wasteful of chemical values but also represents an expense from which no return is realized. Hence, a number of other methods for disposal have been suggested, and some of these have been adopted to a greater or less extent.

The spent pickling liquor, when sulphuric acid has been used as the acidic agent, typically contains 18 to 22 per cent ferrous sulphate plus 8 to 12 percent sulphuric acid in the case of continuous strip picklers, and 20 to 30 per cent ferrous sulphate plus 1 to 5 per cent acid in the case of batch picklers.

The availability of ammonia, as a by-product in the production of coke for metallurgical purposes, in close proximity to iron and steel plants where pickling is done, may be used to advantage in processes wherein the ammonia brought into contact with the spent pickling liquors [and reacts to form ammonium sulphate and oxides or other compounds of iron. The ammonium sulphate may be sold as such, and the iron com- 50 pounds used as a source of iron.

While the process of the present invention is based upon the reaction between ammonia and ferrous sulphate, it presents a number of advantages over processes hitherto known, many of which have not been adopted, or involve many difliculties, because of operating troubles and costsin carrying them out. p

It is, therefore, an object of the present invention to utilize advantageously both spent sulphate pickling liquor and ammonia/cal gas obtained from the production of metallurgical coke.

Another object is to produce ammoniumsulphate of desirable purity, and iron compounds suitable for use, for example, in blast furnaces for production of iron, for the absorption of sulphur compounds in the purification of fuel gases, and for other purposes.

Another object is to provide a process of the type described wherein certain operating difficulties of processes hitherto known-such as the elimination of the gelatinous or flocculent precipitate of ferrous hydroxide, are overcome.

Other objects will be evident from the following disclosure.

In the present disclosure, the term, spent pickling liquor, is intended to mean such liquor obtained by the use of sulphuric acid on iron or steel objects. I

The several steps of the present invention may be summarized as follows: The ammonia and the spent pickling liquor are cont-acted to bring about the formation of ammonium sulphate and compounds of iron. The product obtained is in the form of a gelatinous or flocculent sludge, which will be more or less Watery, depending upon the original water content of the spent pickle liquor and the amount of water loss during the reaction. The sludge is of a gelatinous character, due to the presence of ferrous hydroxide and other ferrous compounds, and is not only incompletely precipitated because of the equilibria existing in aqueous solutions containing ammonia and ferrous salts but is also exceedingly diflicult to handle for direct separation of the iron compounds from the ammonium sulphate, by any processes hitherto known. According to the present invention, however, a relatively simple procedure is followed for sludge treatment. In so doing, I mix the sludge with a suitable solid material, preferably finely divided, in sufiicient quantity to stiffen and largely dry. up the sludge. The resulting sludge-solids mixture will therefore be in the form of moist, finely-divided, friable, solid matter rather than gelatinous. This mixture is then subjected to heat and oxide tion, so that the water is driven off, the iron is converted largely to the ferric form, and the reaction between ammonia and the iron compounds completed to form iron hydrate and ammonium sulphate. The resulting dry pulverulent material is then leached, prefenably countercurrently, to remove the water-soluble ammoniiun sulphate, While the iron compounds, which are insoluble, remain mixed with the finely-divided, solid material. This remaining product is suitable for various uses, such as for charging into a blast furnace or other apparatus for making iron, and steel, for reacting with sulphur compounds in the purification of fuel gases, and for other purposes.

The solid material with which the sludge mixed may be any substance suitable for the purpose, such as cinders, pumice, etc, but its nature should also be governed by the uses to which the product is put. Thus, if the product is used in charging a blast furnace, the said solid material should preferably be some carbonaceous material such as coke breeze, or pulverized coal or charcoal.

The process of the present invention will now be described in detail and more or less by way of example, it being understood that I am not limited to the exact amounts, proportions, and procedures given, and that the process may be varied within the scope of the appended claims.

In reacting the spent pickle liquor with ammonia, it is preferable, although not absolutely necessary, that the water present he reduced to as low a point as is consistent with good operating practice for this invention. With decreased amounts of water, the equilibrium of the reaction between ammonia and ferrous sulphate to form iron hydrate and ammonium sulphate is forced more nearly to completion. Reduction in water content may be obtained by any one of a number of ways. For example, the spent pickling liquor may first be concentrated as by heating; or it may be spray-processed to produce copperas (FGSO4I7H2O) in accordance with the Stevenson U. S. Patent No. 1,515,799, and the copperas may be directly reacted with the ammonia in a rotary reactor; or the hot, spent pickle liquor or hot copperas solution may be atomized into an atmosphere of ammonia, whereby the ferrous sulphate-ammonia reaction takes place simultaneously with loss of water.

Whichever method is used, there results a more or less gelatinous sludge containing ammonium sulphate, generally some uncoinbined ammonia, and water, intermingled with the gelatinous ferrous hydroxide, with or without other compounds of iron.

The next step is to mix this sludge with a suitable comminuted solid material, e. g., coke breeze, charcoal, or pulverized coal, and to treat the mixture in order to remove water and oxidize the iron largely to the ferric state. These procedures may be carried out in the same device, or the mixing with the comminuted solid matter may be done first. As an example of the former, I introduce the sludge and the solid matter separately, or together, into the upper end of an inclined tubular rotary mixing device, of any desired conventional design, and at or near the same point I also introduce the hot oxidizing and drying gas. This gas may conveniently be a mixture of hot flue gas and air, in proper proportions to provide adequate oxidation and suitable temperature, or it may be entirely air heated in some convenient Way, as by being blown through a checkerbrick-filled chamber that is alternately heated with combustion gases. The air, of course, is the source of oxygen for the oxidation. The temperature and volume of the gas should be suflicient to carry off the water in the superheated state, so that it will not condense on the charge further along in the rotary mixing device. On the other hand, the temperature should not be so high as t decompose considerable amounts of the ammonium sulphate and so cause a net loss of ammonia from the charge. For example, I find that an outlet temperature 01 about Silt" F. for the discharged material is convenient for most operating purposes. The temperature of the incoming gases will, of course, be higher, but the presence of water in the aggregate serves generally to prevent any deleterious decomposition at that point. As the water is evaporated, some ammonia goes with it, but this ammonia is practically wholly reabsorbed by the drier and partly oxidized portions of the charge. I have found that the free ammonia present in such a precipitated gelatinous mass, to the degree required for stoichiometrical reaction with the ferrous sulphate or other iron salts unprecipitated at the reaction equilibrium, is desorbed at the wet or inlet end of the drier-oxidizer, but becomes reabsorbed as the drying and oxidation proceeds toward the dry end of the apparatus. Thus, complete reaction is assured and only minor ammonia losses result.

In the foregoing step of drying and oxidizing, it is desirable that the mixing of the sludge and comminuted solid material be substantially complete before the mixture is dried out to any appreciable degree. The sludge deposits upon the coimninuted solid material to make a mass which is not only readily handled in the drying-oxidizing step, but also in subsequent steps of .leaching and further handling. The amount of comminuted solid material used will depend upon the amount of water present in the sludge, and upon the requirements for obtaining good interracial contact between the solids and gases in the drying-oxidizing apparatus. The drying-oxidizing gases should flow concurrently with the movement of the charge, since countercurrent flow would tend to result in loss or" ammonia and other difficulties which would impair the value of the process.

It is desirable to provide in the tubular rotary mixing device suitable mechanical means for agitation, distribution, and mixing,--particularly in the wet (upper) end. These means may include loose balls, rods, chains, etc., as well as scrapers and deflectors. Various other modifications in the mechanical devices may be introduced, such as dust catchers in the flue gas line, screw conveyors, etc. i

The dried, oxidized charge is now ready for the leaching step. This is conveniently carried out by using a countercurrent, semi-continuous operation, wherein a battery f a number of leach boxes is set up and hot water is flowed countercurrently through the boxes from the most exhausted batch to the freshest batch, each box being number one in succession. The liquor flowing out of each last box is a substantially saturated aqueous solution of ammonium sulphate. The contents of each first box, on removal from the battery when exhausted, is substantially entirely the comminuted solid material and iron compounds, together with water. The iron compounds are largely ferric oxide, more or less hydrated.

As a more specific example of the leaching step, I may take an oxidized charge which analyzes about 1000 parts coke breeze, 500 parts ammonium sulphate, 200 parts iron oxide, and parts water, parts being by weight, and leach this in a battery of six vats, of which one will be filling, one dumping, and four in the leaching circuit. The water hold-up per vat is about 20% on the weight of the charge in the vat, and the water necessary to obtain asaturated solution of allthe ammonium sulphate is about on the weight of the same charge. Hence about 50% of water on the weight of the charge in the first (i. e., the most nearly exhausted) vat should be added,-assuming the weight of the contents of each vat to be about the same.

The leach liquor may be treated in any desired manner to recover the ammonium sulphate, which, by the present process, is of a good degree of purity and suitable for use, e. g., in fertilizers, without further purification. Or it may be completely purified if desired. The iron oxide-carbonaceous material mixture may be fed to a blast furnace or other means for making iron, without further treatment, as it is sufiiciently low in impurities, such as sulphur, to make it entirely suitable for such purposes.

In processes hitherto described and used for the preparation of ammonium sulphate by reacting spent pickling liquor with the by-product ammoniacal gases from metallurgical coke manufacture, it has always been considered essential to purify the ammoniacal gases in order to remove sulphur, carbon dioxide, cyanogen, and the like which deleteriously alfected the said processes, or to utilize special means for eliminating the eifects of these impurities. In the process of the present invention, however, I find such purification to be unnecessary. These undesirable components are driven off together with the water vapor in the drying-oxidizing step, at the temperatures employed therein, and they and their compounds are, at least for all practical purposes, absent from the charge after this drying-oxidizing step. Hence the process of the present invention has an added advantage in that preliminary purification of the ammoniacal gases may be dispensed with, thus saving an appreciable amount of time, labor, apparatus, and expense.

This invention has been described with particular emphasis upon its use for recovering the values of both spent pickling liquor and by-product ammonia from metallurgical coke manufacture. It should, however, be obvious that other sources of both ferrous sulphate and ammonia may be employed without departing from the spirit and scope of the present invention.

I claim:

1. Process for making ammonium sulphate and iron oxide, which comprises, reacting together a ferrous sulphate solution and ammonia, to produce a gelatinous sludge, mixing the resulting product with comminuted Water-insoluble solid material to make a Wet, friable, non-gelatinous aggregate, and subjecting said aggregate to the action of a heated, oxygen-containing gas in concurrent motion with said aggregate, for a sufficient time and at a sufiicient temperature to remove the bulk of the water and convert substantially all of the iron compounds to the ferric state, Without appreciable loss of ammonium sulphate during the reaction.

2. Process for making ammonium sulphate and iron oxide, which comprises reacting together a ferrous sulphate solution and ammonia, to produce a gelatinous sludge, mixing the resulting product with comminuted water-insoluble solid material to make a wet, friable, non-gelatinous aggregate, drying out substantially all of the water and oxidizing the bulk of the iron compounds to the ferric state by means of a heated, oxidizing gas moving concurrently with said aggregate, controlling the temperature and time of treatment with said gas to effect said oxidizing while avoiding appreciable decomposition and loss of reactants other than water, and then subjecting the resulting product to counter-current leaching with water to remove the ammonium sulphate.

3. Process for making ammonium sulphate and iron oxide, which comprises reacting together a ferrous sulphate solution and ammonia, to produce a gelatinous sludge, mixing the resulting product with comminuted water-insoluble solid material to make a wet, friable, non-gelatinous aggregate, and subjecting said aggregate to the action of a heated, oxygen-containing gas in concurrent motion with the aggregate, under such conditions of temperatures as to avoid appreciable loss of ammonia and ammonium sulphate from said aggregate, and for a sufficient time and at a sufficient temperature to remove the bulk of the water and oxidize the iron compounds to the extent that substantially no soluble ferrous compounds are present and the greater part of the iron is converted to the ferric state.

4. In a process for preparing ammonium sulphate, the steps which comprise forming a gelatinous reaction product of a ferrous sulphate solution and ammonia, mixing said product with an inert comminuted water-insoluble solid material to give a friable, substantially non-gelatinous aggregate, and oxidizing and drying said aggregate by subjecting the same to treatment in concurrent motion With a drying, oxygen-containing gas, and controlling the temperature and time of treatment with said gas to eifect oxidation of the iron compounds to the extent that substantially no soluble ferrous compounds are present and the greater part of the iron is converted to the ferric state, while avoiding appreciable loss of ammonia and ammonium sulphate from said aggregate.

5. Process accordingto claim 4, wherein the gas is a mixture of air and hot flue gas.

6. Process for making ammonium sulphate, which comp-rises reacting togethera solution of ferrous sulphate and ammonia to produce a gelatinous sludge, mixing the resulting gelatinous product with inert comminuted water-insoluble solid material to make a friable, non-gelatinous aggregate, and oxidizing and drying said aggregate by subjecting the same to treatment in concurrent motion with a drying, oxygen-containing gas, without appreciable'loss of ammonium sulphate during the reaction and for a sufficient time and at a sufficient temperature to oxidize substantially all of the iron compounds to the ferric state.

'1. Process according to claim 1, wherein the temperature of the dried and oxidized aggregate at the end of the heating and oxidizing step is approximately 300 F.

8. Process according to claim 1, wherein the heated oxidizing gas is a mixture of air and hot fiue gas.

9. In a process for recovering values from spent sulphate pickling liquors, the steps which comprise reacting said liquors and an ammoniacal gas to produce a gelatinous reaction product composed practically entirely of ammonium hydroxide, ammonium sulphate, iron oxide and other iron compounds substantially wholly in the ferrous state, and water, mixing said gelatinous reaction product and an inert comminuted waterinsoluble solid material to form a wet, friable, substantially non-gelatinous aggregate, subjecting said aggregate to a stream of heated oxidizing gas,

compounds to the extent that substantially no soluble ferrous compounds remain in the treated product and the greater part of the iron is converted to the ferric state.

LEROY F. MAREK. 

